Process for the production of vinyl acetate

ABSTRACT

A process for the production of vinyl acetate which comprises contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps of (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium, and (c) sintering the supported palladium at a temperature of greater than 500 DEG  C.

The present invention relates to a process for the production of vinylacetate by contacting ethylene, acetic acid and an oxygen-containing gaswith a supported palladium catalyst.

The preparation of supported palladium catalysts for the production ofvinyl acetate generally involves impregnating a suitable support with apalladium compound followed by conversion of the palladium compound tosubstantially metallic palladium.

Methods for the preparation of shell-impregnated catalysts aredescribed, for example, in U.S. Pat. No. 3,822,308, U.S. Pat. No.4,048,096, U.S. Pat. No. 5,185,308, U.S. Pat. No. 5,332,710, CA 2128162,U.S. Pat. No. 4,087,622, CA 2128154, CA 2128161 and U.S. Pat. No.5,422,329.

Methods for the preparation of non-shell type catalysts are describedin, for example, U.S. Pat. No. 3,743,607, GB 1333449, U.S. Pat. No.3,939,199, U.S. Pat. No. 4,668,819, EP 330853, EP 403950, EP431478 andCA 2071698.

U.S. Pat. No. 5,336,802 describes a method for the pre-treatment ofpalladium-gold catalysts in which the catalyst is heated in the presenceof an oxidising agent such as air at a temperature at least sufficientto partially oxidise the palladium; the oxidising agent is withdrawn andan inert gas such as nitrogen is introduced; the catalyst is then heatedagain at a temperature up to 500° C. in the presence of a reducing agentsuch as hydrogen or ethylene. The process described therein isillustrated with a "conventional catalyst containing nominally 1%palladium and 0.5% gold".

It is known that the activity for vinyl acetate production of supportedpalladium catalysts declines with use. If the catalyst's activity andhence the process productivity declines to a commercially unacceptablelevel, it is necessary to regenerate and/or replace the catalyst.Deactivation of vinyl acetate catalysts is described by Abel et al. inChem. Eng. Technol. 17 (1994) 112-118.

Merely increasing the amount of palladium in the catalyst to increasethe lifetime of the catalyst presents a problem in that the initialactivity of the catalyst may be too high for safe and/or controllableoperation on an industrial scale, for example, due to the limited heatremoval capacity of the plant.

There remains a need for a process for the preparation of a supportedpalladium catalyst for use in the production of vinyl acetate whichovercomes this problem.

Thus, according to the present invention, there is provided a processfor the production of vinyl acetate which process comprises contactingethylene, acetic acid and an oxygen-containing gas with a supportedpalladium catalyst prepared by a process comprising the steps: (a)impregnating a catalyst support with a palladium compound, (b)converting the palladium compound to substantially metallic palladium,and (c) sinterinig, the supported palladium at a temperature of greaterthan 500° C.

The present invention solves the technical problem defined above bysintering the palladium on the support at a temperature of greater than500° C.

Without wishing to be bound by any theory it is believed that thissintering step causes palladium metal particle growth which decreasesthe initial activity of the catalyst. Thus, catalysts having, a highpalladium concentration but a commercially acceptable initial activitymay be prepared by the process according to the present invention andsuch catalysts have a longer commercially useful life than conventionalcatalysts. The sintering step also increases the average pore size ofsilica supports. The catalysts of the present invention have also beenfound to be less susceptible to the adverse effects of excessconcentration of promoter such as potassium acetate.

The sintering step (c) is preferably performed using a reducing gas, butcan be performed in the presence of an oxidising gas or in an inert gas.Suitable reducing gases are hydrogen and carbon monoxide. A suitableoxidising gas is oxygen. These may be diluted with an inert gas.Suitable inert gases for use alone or in conjunction with oxidising orreducing gases are nitrogen, carbon dioxide and helium. Suitabletemperatures for the sintering step are from greater than 500 to 1000°C. with preferred temperatures being in the range 650-1000° C. Preferredtimes for the sintering step are between 1 and 24 hours. If an oxidisinggas is used then the catalyst needs to be subsequently reduced. Thecatalyst can be purged with an inert gas prior to sintering and duringthe heat-up period (for safety) and during cool-down (to less than 100°C., more preferrably to less than 60° C.) to prevent any redispersion ofthe palladium. Any suitable or practicable heat-up and cool-down ratescan be used. The sintering step (c) on a commercial scale can beperformed in a tower or vessel capable of fulfilling the processconditions outlined above. The catalyst can be agitated by the gas flowduring the process. A rotary screw furnace can be used. On thelaboratory scale, a horizontal or vertically mounted tube in an electricfurnace can be used provided that gas-solid contact is efficient(length/diameter will need to be considered). Pre-heating of the gasstream may be required. The time and temperature of the sintering stepare related; the higher the temperature, the shorter the time required.Those skilled in the art will be able to adapt these parameters to fitthe scale of operations. Typically the sintering step (c) causespalladium metal particle growth from 3-4 nm in diameter to 8-15 nm indiameter.

The conversion of the palladium compound to substantially metallicpalladium in step (b) may be achieved by a reduction step which canimmediately precede the sintering step (c) and by performing the twoprocess steps in the same equipment.

The catalyst preparation process of the present invention may be usedfor the preparation of uniformly impregnated or shell impregnatedcatalysts, for use in fluid bed or fixed bed processes for theproduction of vinyl acetate.

The catalyst preparation process of the present invention may be used toprepare catalysts having high palladium concentrations, for examplegreater than 0.5% by weight, preferably greater than 1% by weight basedupon the total weight of the catalyst The palladium concentration may beas high as 5% by weight for fluid bed or as high as 10% by weight forfixed bed applications. The initial activity of a supported palladiumcatalyst having high palladium concentration, if prepared by aconventional process, would be expected to be very high and might evenbe so high as to be unsafe and/or uncontrollable if used on a commercialscale. However, when prepared by the process of the present intentional,the initial activity of the catalyst is reduced compared to that of aconventionally prepared catalyst, whereas the high palladiumconcentration results in commercially acceptable activity for theextended lifetime of the catalyst.

For the preparation of both shell impregnated and uniformly impregnatedcatalysts, suitable catalyst supports may comprise porous silica,alumina, silica/alumina, titania, zirconia or carbon, preferably silica.Suitably, the support may have a pore volume from 0.2 to 3.5 ml per gramof support, a surface area of 5 to 800 m² per gram of support and anapparent bulk density of 0.3 to 1.5 g/ml. For catalysts used in fixedbed processes the support typically has dimensions of 3 to 9 mm. Forcatalysts used in fixed bed processes the support typically may bespheric, tablet, extrudate, pill shaped or any suitable shape. Forcatalysts used in fluid bed processes the support typically may have aparticle size distribution such that at least 60% of the catalystparticles have a particle diameter of below 200 microns, preferably atleast 50% less than 105 microns and no more than 40% of the catalystparticles have a diameter less than 40 microns.

In step (a) the support is preferably impregnated with a palladiumcompound in a suitable solvent. Suitable solvents may be water,carboxylic acids such as acetic acid, benzene, toluene, alcohols such asmethanol or ethanol, nitriles such as acetonitrile or benzonitrile,tetrahydrofuran or chlorinated solvents such as dichloromethane.Preferably, the solvent is water and/or acetic acid. Suitably, thesupport is impregnated with palladium acetate, sulphate, nitrate,chloride or halogen-containing palladium salts such as H₂ PdCl₄, Na₂PdCl₄ or K₂ PdCl₄. A preferred water soluble compound is Na₂ PdCl₄. Apreferred acetic acid-soluble palladium compound is palladium acetate.

The impregnation of the support may be performed by dipping, immersionor spraying the support in contact with a solution of the palladiumcompound. The impregnation may be performed in one or more steps or in acontinuous process. The support may be contacted with the impregnatingpalladium solution by tumbling, rotating, swirling or a similar process,to give uniform impregnation. The impregnation is typically performed atambient temperature. Elevated temperatures may be used for example, withpalladium acetate in acetic acid, up to 120° C., preferably up to 100°C., more preterably up to 60° C. Impregnation is performed carefully soas to avoid the break up or attrition of the support. The support can befilled up by the impregnating solution to 5-100% of the pore volume.

In addition to palladium compounds the support may also be impregnatedin step (a) with gold, copper and/or nickel compounds, preferably gold,which are converted to the metal along with the palladium in step (b)and are present as mixtures and/or alloys with the palladium in themetallic palladium particles. Suitable gold compounds include goldchloride, tetrachloroauric acid (HAuCl₄), NaAuCl₄, KAuCl₄, dimethyl goldacetate, barium acetoaurate or gold acetate, preferably HAuCl₄. Thesepromoters may be used in an amount of 0.1 to 10% by weight of eachpromoter metal present in the finished catalyst.

In addition to palladium and optional gold, copper and/or nickel thesupport may also be impregnated at any suitable stage during thepreparation process with one or more salts of Group I, Group II,lanthanide or transition metals, preferably of cadmium, barium,potassium, sodium, iron, manganese, nickel, antimony and/or lanthanum,which are present in the finished catalyst as salts, typically acetates.Generally potassium will be present. Suitable salts of these compoundsare acetates or chlorides but any soluble salt may be used. Thesepromoters may be used in an amount of 0.1 to 15%, preferably 3 to 9%, byweight of each promoter salt present in the finished catalyst.

The impregnated support may optionally be dried and the impregnatingstep repeated two or more times if higher palladium or promoterloadings, than the solubility of the salt in the solvent will allow, arerequired. The drying step may be performed at up to 120° C., preferablyup to 100° C., and most preferably at 60° C. The drying step may beperformed at ambient temperature and reduced pressure. Air, nitogen,helium, carbon dioxide or any suitable inert gas may be used in thedrying step. The catalyst may be tumbled, rotated or agitated by the gasstream to aid drying.

To prepare shell impregnated catalysts the wet or dry impregnatedsupport is contacted with a base solution with swirling, tulimbling,rotation, mixing or the like. The base solution can also be applied byspraying onto the impregnated support during tumbling, rotation, mixingor the like. Bases can be Group I or II hydroxides, carbonates orsilicates. Typical examples are sodium hydroxide, sodium metasilicate,potassium hydroxide, potassium metasilicate and barium hydroxide. Thebase solution can be applied in one or more steps with suitable timedelays between applications. The temperature of the precipitation stepis usually ambient but can be raised up to 100° C. Any solvent can beused in which the base material is soluble, water is preferred. The baseshould be contacted with the impregnated support for a suitable periodso that the metal salts are precipitated in a shell. This usually takesgreater than one hour, preferrably between 8 to 24 hours. An optimalamount of base will be required for the precipitation and is usuallyrequired in excess, commonly this is 1.8 times the notional amountrequired to generate the hydroxides of the metal salts.

The impregnated support can be washed to remove anion contaminants, forexample, nitrates, sulphates and usually halides. For chloride removal,washing with de-ionised water should proceed until a silver nitrate testshows that there is no chloride present. The anion contamination levelsshould be minimised. Cation contaminants should be minimised, forexample to below 0.5 wt %, preferably below 0.2 wt % of sodium in thedried catalyst. Low levels of these contaminants are likely to remain,it is not essential that the levels are absolutely zero. On a commercialscale, batch washing may be used. To speed up the process, warm watermay be used. Also, ion exchange solutions (such as potassium acetate)can be used to displace chloride and sodium. Also, the reagents used forthe preparation can be selected to avoid the use of chloride and sodium,for example, potassium metasilicate instead of a sodium salt.

In step (b) the palladium compound can be converted to metal before orafter the optional washing step above, depending on the reagents used.Liquid reducing agents such as aqueous hydrazine, formaldehyde, sodiumformate, methanol or alcohols, preferably aqueous hydrazine can be used.Reduction may also be performed with gases such as carbon monoxide,hydrogen and ethylene. These can be diluted with an inert gas such asnitrogen, carbon dioxide or helium. Typically, the gaseous reductiontakes place at elevated temperatures of 100-500° C. until the materialis reduced. Typically, reduction in the liquid reducing agents takesplace and ambient temperatures but temperatures up to 100° C. may beused.

After the palladium has been converted to metal it is sintered as hereindescribed. The sintering step (c) may follow on from the step (b) byfurther heating the catalyst in the reducing gas to greater than 500° C.The material may then be impregnated with promoter salts as here beforedescribed.

The ethylene, acetic acid and oxygen-containing gas may be contactedwith the supported palladium catalyst prepared according to the catalystpreparation process of the present invention by methods known in theart. Thus, the reactants may be contacted with the catalyst in a fixedbed or a fluid bed at temperatures in the range 145 to 195° C. andpressures in the range 1 atm to 20 atm. The vinyl acetate product may berecovered by conventional methods known in the art.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be illustrated by reference to FIGS. 1 to 3 andthe following examples and experiments. FIG. 1 is a schematicrepresentation of some of the possible catalyst preparation methodsaccording to the present invention. FIG. 2 is a graph comparingproductivity as a function of time of a catalyst prepared according tothe invention to that of a catalyst not prepared according to theinvention. FIG. 3 is a graph comparing the effect of the amount ofpromoter potassium acetate has on the activity of a catalyst preparedaccording to the invention to that of catalysts not prepared accordingto the invention.

Referring to FIG. 1 uniform (non-shell) type catalysts may be preparedby the steps of impregnating a support with palladium salts and optionalpromoters followed by drying and reduction of the metals. The materialmay then be optionally washed and dried before sintering according tothe present invention and final impregnation with optional promoterssuch as acetates of potassium, sodium, cadmium or barium.

To prepare shell type catalysts the support impregnated with palladiumand optional promoters such as gold may be optionally dried. The metalsare then precipitated. The material may then be passed to either (i)reduction to metals, washing and drying, or (ii) washing and dryingfollowed by reduction to metals. The material is then subjected tosintering according to the present invention followed by impregnationwith promoters such as acetates of potassium, sodium, cadmium or barium.

EXAMPLE 1

Catalyst A was prepared according to the present invention to have anotional composition (that is without allowing for any losses duringpreparation) of 1.8% by weight palladiun, 0.8% by weight gold and 7% byweight potassium acetate.

1. Impregnation of Support.

15 g of KA160 silica support spheres (4-6 mm, SudChemie) were added to asolution of 1.0264 of sodium tetrachloropalladate trihydrate (JohnsonMatthey) and 0.2655 g of chloroauric acid trihydrate (Aldrich) in 9.1 gof de-ionised water. The addition was done in one portion and themixture swirled until all the solution had been absorbed evenly. Theimpregnated support was then allowed to stand covered for two hours atroom temperature.

2. Precipitation of Palladium and Gold Compounds on Support.

A solution of 1.7 g sodium metasilicate pentahydrate (Fisons) in 18 g ofwater was added to the impregnated support from step 1. The mixture wasswirled briefly a few times over 15 minutes to avoid the formation of"spots" and then permitted to stand undisturbed overnight.

3. Reduction of Palladium and Gold to Substantially Metallic State.

The aqueous phase above the material from step 2 was treated with 5 g of55% hydrazine hydrate (Aldrich).

4. Washing of Supported Compounds.

The aqueous phase was decanted off and the material from step 3 waswashed four times with about 50 ml of water decanting after each wash.The resultant material was transferred to a glass column fitted with astopcock and then washed with de-ionised water at approximately 1 literper 12 hours until a silver nitrate test proved negative. The materialwas dried at 60° C. overnight in a forced air oven and cooled.

5. Sintering of Palladium (and Gold)

The supported palladium material from step 4 was transferred to ahorizontally mounted furnace and packed into the centre of a quartz tubeliner with quartz wool and KA160 support (previously dried thoroughly)filling the void space. The quartz tube liner was placed inside a steeltube and gas supplies connected. The furnace temperature was raised to150° C. at 10° C./min and maintained at this temperature for 2 hoursunder a constant stream of nitrogen. Hydrogen flow at a GHSV of 60/hrwas commissioned and the nitrogen flow stopped. The furnace temperaturewas raised to 800° C. at 30° C./min and maintained at this temperaturefor 11 hours. After this period the resultant material was allowed tocool to room temperature under hydrogen flow. Nitrogen flow wasre-commenced and hydrogen flow stopped before discharging the material.

6. Metal Acetate Impregnation

The dry material from step 5 was impregnated with 1.16 g of anhydrouspotassium acetate (Aldrich) dissolved in 8.8 g of water. The mixture wasswirled gently until the liquid was absorbed. The resultant material wasdried again overnight at 60° C.

EXAMPLE 2 (COMPARATIVE)

Catalyst B was prepared according to the procedure of Example 1 exceptthat the sintering step 5 was omitted.

EXAMPLE 3 (COMPARATIVE)

Catalyst C was prepared according to the procedure of Example 1 exceptthat the sintering step 5 was omitted and metal loadings were reduced togive the same initial activity as the catalyst prepared in Example 1.

Catalyst Testing in Microrector

The catalysts prepared above were tested in a microrector using thefollowing general procedure. The tests were performed at 7.8 barg and150° C. using catalyst pellets (prepared as above, amount specified inTable 1) diluted with 60 ml of 1 mm glass beads and loaded into astainless steel tube of internal diameter 10-11 mm. The catalyst wascommissioned at 7.8 barg by heating at 160° C. for 3 hours in a streamof nitrogen and then 150° C. in a stream of ethylene. Acetic acid vapourwas then mixed with the ethylene and passed over the catalyst for aperiod of at least 50 minutes. A mixture of 21% oxygen in helium wasgradually added to the feed gas while maintaining the maximum catalystbed temperature at 150° C. The catalyst hot spot vas maintained at 150°C. The final composition of the reactant mixture was ethylene: aceticacid: oxygen: helium=53.1:10.4:7.7:28.6 by volume and the total gashourly space velocity was 3850 hr⁻¹. The product stream was analysed inthe vapor phase at hourly intervals by means of an on-line gaschromatograph.

Activity of the catalyst was calculated as grams of vinyl acetateproduced per liter of catalyst per hour (space time yield, STY) and theselectivity of the catalyst was calculated as the percentage ofconverted ethylene present in the product. Data is reported on the basisof the average of the activities and selectivities measured between 17and 22 hours after full oxygen content was achieved.

The results, comparing the activities of catalyst A, B and C, arepresented in Table 1.

                  TABLE 1                                                         ______________________________________                                                     Catalyst                                                                        loaded     Activity                                                           in micro-                                                                             (grams of vinyl                                                       reactor   acetate per hour                                                                               Selectivity                                Catalyst                                                                              (grams)   per liter of catalyst)                                                                   (%)                                       ______________________________________                                        Example 4                                                                              A       2.5       715        92.4                                    Example 5                                                                                           2.0      1381                    90.7                   (comparative)                                                                 Example 6                                                                                           2.5      718                      93.4                  (comparative)                                                                 ______________________________________                                    

A comparison of the activities of catalysts A and B in Table 1 showsthat the sintering step (step 5) caused the activity of catalyst A to bedecreased. This is consistent with (growth of palladium particle sizeand loss of palladium metal surface area. Catalyst C was prepared withlower metal loadings than catalysts A and B; the metal loadings beingselected to give the same initial activity as catalyst A. Catalysts Aand C would thus be expected to have similar intial operationalbehavior. It would however be expected that catalyst A would maintainproductivity for a longer period than catalyst C if palladium particlegrowth and loss of palladium metal surface area are the cause of reducedintial activity. This is illustrated in Examples 7 and 8.

Testing of Catalysts in Larger Reactors

Catalysts A and C were tested in larger tubular reactors as follows.77.5 g of catalyst A (Example 7) and 77.5 of catalyst C (Example 8comparative) were each loaded into separate 6 foot reactor tubes. Thesetwo tubes were placed in the same fluidised bed sand bath. The bathtemperature could be controlled and each tube had its own gas/liquidfeed and product handling, system. Nitrogen flow was commenced at 1106ml/min (@STP) and ethylene flow at 2590 mil/min (@STP). The sand bathand tubes were heated to 150° C. and the reactor pressure was raised to115 psig. Acetic acid flow at 155 g/hr (containing 2 wt % water) wascommenced to a vapouriser and mixed with nitrogen and ethylene. A smallstream of acetic acid (2 wt % water, 0.0285 wt % potassium acetate) at13 g/hr was introduced to the preheater zone to be vaporised with themain gas stream. After a few hours oxygen was commenced at 153 ml/min(@STP). The product stream was analysed by on-line gas chromatographyand then condensed to give a crude liquid product of vinyl acetate,acetic acid and water, the remaining gases were vented and sampled bythe on-line gas chromatography. Vinyl acetate production was monitoredfor both catalysts. As the catalysts deactivated a constant productionrate was initially maintained by gradually increasing the oxygen feed toa maximum level of 425 ml/min (@STP). At full oxygen flow the gas feedcomposition was ethylene:aceticacid:water:oxygen:nitrogen=49.7:19.6:1.3:8.2:21.2 by volume at a totalGSHV of 2261 hr⁻¹ (@STP). Once full oxygen feed rate was achieved,constant production on catalyst A was further maintained by graduallyincreasing the sand bath temperature from approximately 150 to 160° C.Since both tubes were in the same sand bath catalyst C production fellbelow that of catalyst A as it deactivated more rapidly. FIG. 2 showsthe normalised daily production for catalysts A and C as a function ofdays on stream. FIG. 2 clearly demonstrates that although the initialproduction capability of the two catalysts was similar, after five dayson stream, the productivity of the comparative catalyst, catalyst C, wasbelow that of the catalyst of the invention, catalyst A. Examination ofthe slopes of the productivities of the two catalysts shows thatcatalyst A maintained production at approximately 1 whereas catalyst C'sproductivity slowly declined with time, ending up at a productivity of0.7. Towards the end of the run the production capabilities of catalystA were tested relative to catalyst C by adjusting the oxygen feed levelsand/or sand bath temperature. The production is seen to alter upwardsand downwards accordingly in FIG. 2 and it is noted that catalyst Aalways has a higher productivity than catalyst C. Catalyst A hasexhibited a slower deactivation rate than catalyst C even though theirinitial activities were very similar.

EXAMPLE 9--FURTHER CATALYST TESTS USING MICROREACTOR

Two further batches of catalyst were prepared according to the procedureof Example 1 except that the quantities of reagents used were scaled bya factor of 9. After the washing and drying step each catalyst batch wasdivided accurately into 9 equal portions and impregnated with the targetwt % potassium acetate loadings (see Table 2). These catalyst sampleswere tested according to the procedure of Examples 1 to 3. FIG. 3 showsthe activity achieved by these catalyst samples and compares it to theactivity of the corresponding catalysts reported in U.S. Pat. No.5,179,056 (this activity is extrapolated according to the modeldescribed in U.S. Pat. No. 5,179,056). FIG. 3 shows that that thecatalyst according to the present invention requires a minimum level ofapproximately 1.5 wt % potassium to be effective whereas for thecatalyst of the patent U.S. Pat. No. 5,179,056, a maximum in activitywas achieved at approximately 2.5 wt % potassium. For the catalystaccording to the present invention, the effect of the promoter isapproximately constant fion approximately 1.5 to 5 wt % potassium. Forthe catalyst according to U.S. Pat. No. 5,179,056 activity begins tofill as promoter loadings are increased.

                  TABLE 2                                                         ______________________________________                                                        Activity                                                      Target    Potassium                                                                           (g Vinyl                                                      (wt. %)                                                                                (wt. %)                                                                              Acetate/               Activity                                                                       Activity                              Potassium                                                                            by XRF    hr/l of  Selectivity                                                                            according                                                                         as in U.S.                             Acetate                                                                                method  catalyst)                                                                             (%)    to model                                                                              5179056                               ______________________________________                                        0      0.12      11      1       13                                           1        0.5      147     74.5    142                                         2        0.87    385      89.1    382                                         3        1.17    548      91        556                                       3        1.2      554     90.3    570                                         4        1.54    710      92        687                                                                                688                                  5        1.86    745      92.5    735    731                                  5        1.9      752     91.4    738    735                                  6        2.1      729     92.8    749    751                                  7        2.6      750     91.8    757    759                                  8        2.8      764     92.8    757    748                                  9        3.2      771     93.1    757    706                                  9        3.3      778     91.5    757    691                                  10     3.6      745      93.3     757    639                                  11     3.8      776      92.1     757    599                                  14     4.8      719      93.3     757                                         15     5.1      759      93         757                                       15     5.2      743      91.8     757                                         ______________________________________                                    

This shows that the catalyst prepared according to the present inventionis more tolerant of excessive potassium acetate promoter concentrations.

We claim:
 1. A process for the production of vinyl acetate which processcomprises contacting ethylene, acetic acid and an oxygen-containing gaswith a supported palladium catalyst prepared by a process comprising thesteps: (a) impregnating a catalyst support with a palladium compound,(b) converting the palladium compound to substantially metallicpalladium, and (c) sintering the supported palladium at a temperature ofgreater than 500° C. in the presence of a gas consisting essentially ofa reducing gas, an inert gas or a mixture thereof.
 2. A process asclaimed in claim 1 wherein the catalyst support is impregnated with apalladium compound in a solvent selected from water, carboxylic acid,benzene, toluene, alcohol, nitriles, tetrohydrofuran or a chlorinatedsolvent.
 3. A process as claimed in claim 2 wherein the solvent is waterand/or acetic acid.
 4. A process as claimed in claim 1 wherein thepalladium compound is palladium acetate, sulphate, nitrate, chloride ora halogen-containing palladium salt.
 5. A process as claimed in claim 4wherein the palladium compound is palladium acetate.
 6. A process asclaimed in claim 1 in which step (b) is carried out by contacting thepalladium compound with a liquid or gaseous reducing agent selected fromthe group consisting of aqueous hydrazine, formaldehyde, sodium formate,alcohol, carbon monoxide, carbon dioxide, hydrogen and ethylene.
 7. Aprocess as claimed in claim 1 wherein step (c) is carried out at atemperature in the range of from 650 to 1000° C.
 8. A process as claimedin claim 1 wherein in step (c) the reducing gas is selected from thegroup consisting of hydrogen and carbon monoxide and the inert gas isselected from the group consisting of nitrogen, carbon dioxide andhelium.
 9. A process as claimed in claim 1 wherein the palladiumcatalyst comprises at least 0.5% by weight palladium based upon thetotal weight of the catalyst.
 10. A process as claimed in claim 1wherein the catalyst support comprises porous silica, alumina,silica/alumina, titania, zirconia or carbon.
 11. A process as claimed inclaim 1 wherein the palladium support is impregnated in step (a) withgold, copper, nickel, one or more salts of Group I, II, Lanthanide ortransition metals.
 12. A process as claimed in claim 1 wherein theethylene, acetic acid and oxygen-containing gas is contacted with thecatalyst at a temperature in the range of 145 to 195° C. and a pressureof from 1 to 20 atmospheres.
 13. A process as claimed in claim 4 inwhich step (b) is carried out by contacting the palladium compound witha liquid or gaseous reducing agent selected from the group consisting ofaqueous hydrazine, formaldehyde, sodium formate, alcohol, carbonmonoxide, carbon dioxide, hydrogen and ethylene.
 14. A process asclaimed in claim 4 wherein step (c) is carried out at a temperature inthe range of from 650 to 1000° C.
 15. A process as claimed in claim 6wherein step (c) is carried out at a temperature in the range of from650 to 1000° C.
 16. A process as claimed in claim 7 wherein in step (c)the reducing gas is selected from the group consisting of hydrogen andcarbon monoxide and the inert gas is selected from the group consistingof nitrogen, carbon dioxide and helium.
 17. A process as claimed inclaim 14 wherein in step (c) the reducing gas is selected from the groupconsisting of hydrogen and carbon monoxide and the inert gas is selectedfrom the group consisting of nitrogen, carbon dioxide and helium.
 18. Aprocess as claimed in claim 1 wherein the time for the sintering step(c) is between 1 and 24 hours.
 19. A process as claimed in claim 7wherein the time for the sintering step (c) is between 1 and 24 hours.20. A process as claimed in claim 8 wherein the time for the sinteringstep (c) is between 1 and 24 hours.
 21. A process as claimed in claim 16wherein the time for the sintering step (c) is between 1 and 24 hours.22. A process as claimed in claim 9 wherein the palladium catalystcomprises up to 10% by weight based upon the total weight of thecatalyst.
 23. A process as claimed in claim 4 wherein the catalystsupport is impregnated in step (a) with gold, copper, nickel, one ormore salts of Group I, II, Lanthanide or transition metals.
 24. Aprocess as claimed in claim 22 wherein the catalyst support isimpregnated in step (a) with gold, copper, nickel, one or more salts ofGroup I, II, Lanthanide or transition metals.
 25. A process as claimedin claim 1 in which the sintering step (c) causes palladium metalparticle growth from 3-4 nm in diameter to 8-15 nm in diameter.
 26. Aprocess as claimed in claim 7 in which the sintering step (c) causespalladium metal particle growth from 3-4 nm in diameter to 8-15 nm indiameter.
 27. A process as claimed in claim 8 in which the sinteringstep (c) causes palladium metal particle growth from 3-4 nm in diameterto 8-15 nm in diameter.
 28. A process as claimed in claim 11 in whichthe sintering step (c) causes palladium metal particle growth from 3-4nm in diameter to 8-15 nm in diameter.
 29. A process as claimed in claim18 in which the sintering step (c) causes palladium metal particlegrowth from 3-4 nm in diameter to 8-15 nm in diameter.
 30. A process asclaimed in claim 21 in which the sintering step (c) causes palladiummetal particle growth from 3-4 nm in diameter to 8-15 nm in diameter.31. A process as claimed in claim 22 in which the sintering step (c)causes palladium metal particle growth from 3-4 nm in diameter to 8-15nm in diameter.
 32. A process for the production of vinyl acetate whichprocess comprises contacting ethylene, acetic acid and anoxygen-containing gas with a supported palladium catalyst prepared by aprocess comprising the steps: (a) impregnating a catalyst support with apalladium compound, and gold, copper, nickel, one or more salts of GroupI, II, Lanthanide or transition metals, (b) converting the palladiumcompound to substantially metallic palladium, and (c) sintering thesupported palladium at a temperature of greater than 500° C. for a timeof between 1 and 24 hours in the presence of a gas consistingessentially of a reducing gas, an inert gas or a mixture thereof whereinthe reducing gas is selected from the group consisting of hydrogen andcarbon monoxide and the inert gas is selected from the group consistingof nitrogen, carbon dioxide and helium.
 33. A process as claimed inclaim 32 wherein step (c) is carried out at a temperature in the rangeof form 650 to 1000° C.
 34. A process as claimed in claim 32 in whichthe sintering step (c) causes palladium metal particle growth from 3-4nm in diameter to 8-15 nm in diameter.
 35. A process as claimed in claim32 wherein the ethylene, acetic acid and oxygen containing gas arecontact with the supported palladium catalyst at a temperature in therange of 145 to 195° C. and a pressure of from 1 to 20 atmospheres. 36.A process as claimed in claim 34 wherein the ethylene, acetic acid andoxygen containing gas are contact with the supported palladium catalystat a temperature in the range of 145 to 195° C. and a pressure of from 1to 20 atmospheres.